Reference

General information

Initialization

bjam's first job upon startup is to load the Jam code that
implements the build system. To do this, it searches for a file
called boost-build.jam, first in the invocation directory, then
in its parent and so forth up to the filesystem root, and finally
in the directories specified by the environment variable
BOOST_BUILD_PATH. When found, the file is interpreted, and should
specify the build system location by calling the boost-build
rule:

rule boost-build ( location ? )

If location is a relative path, it is treated as relative to
the directory of boost-build.jam. The directory specified by
that location and the directories in BOOST_BUILD_PATH are then searched for
a file called bootstrap.jam, which is expected to
bootstrap the build system. This arrangement allows the build
system to work without any command-line or environment variable
settings. For example, if the build system files were located in a
directory "build-system/" at your project root, you might place a
boost-build.jam at the project root containing:

boost-build build-system ;

In this case, running bjam anywhere in the project tree will
automatically find the build system.

The default bootstrap.jam, after loading some standard
definitions, loads two site-config.jam and user-config.jam.

Builtin rules

This section contains the list of all rules that
can be used in Jamfile—both rules that define new
targets and auxiliary rules.

Creates an object file. Useful when a single source
file must be compiled with special properties.

glob

The glob rule takes a list shell pattern
and returns the list of files in the project's source directory that
match the pattern. For example:

lib tools : [ glob *.cpp ] ;

It is possible to also pass a second argument—the list of
exclude patterns. The result will then include the list of
files patching any of include patterns, and not matching any
of the exclude patterns. For example:

lib tools : [ glob *.cpp : file_to_exclude.cpp bad*.cpp ] ;

glob-tree

The glob-tree is similar to the
glob except that it operates recursively from
the directory of the containing Jamfile. For example:

ECHO [ glob-tree *.cpp : .svn ] ;

will print the names of all C++ files in your project. The
.svn exclude pattern prevents the
glob-tree rule from entering administrative
directories of the Subversion version control system.

Assigns a symbolic project ID to a project at
a given path. This rule must be better documented!

explicit

The explicit rule takes a single
parameter—a list of target names. The named targets will
be marked explicit, and will be built only if they are explicitly
requested on the command line, or if their dependents are built.
Compare this to ordinary targets, that are built implicitly when
their containing project is built.

always

The always funciton takes a single
parameter—a list of metatarget names. The top-level targets produced
by the named metatargets will be always considered out of date. Consider this example:

exe hello : hello.cpp ;
exe bye : bye.cpp ;
always hello ;

If a build of hello is requested, then the binary will
always be relinked. The object files will not be recompiled, though. Note that if
a build of hello is not requested, for example you specify just
bye on the command line, hello will not
be relinked.

constant

Sets project-wide constant. Takes two
parameters: variable name and a value and makes the specified
variable name accessible in this Jamfile and any child Jamfiles.
For example:

constant VERSION : 1.34.0 ;

path-constant

Same as constant except that
the value is treated as path relative to Jamfile location. For example,
if bjam is invoked in the current directory,
and Jamfile in helper subdirectory has:

path-constant DATA : data/a.txt ;

then the variable DATA will be set to
helper/data/a.txt, and if bjam
is invoked from the helper directory, then
the variable DATA will be set to
data/a.txt.

build-project

Cause some other project to be built. This rule
takes a single parameter—a directory name relative to
the containing Jamfile. When the containing Jamfile is built,
the project located at that directory will be built as well.
At the moment, the parameter to this rule should be a directory
name. Project ID or general target references are not allowed.

test-suite

This rule is deprecated and equivalent to
alias.

Builtin features

This section documents the features that are built-in into
Boost.Build. For features with a fixed set of values, that set is
provided, with the default value listed first.

variant

A feature combining several low-level features, making it easy to
request common build configurations.

Users can define their own build variants using the
variant rule from the common module.

Note: Runtime debugging is on in
debug builds to suit the expectations of people used to various
IDEs.

link

Allowed values:shared,
static

A feature controling how libraries are built.

runtime-link

Allowed values:shared,
static

Controls if a static or shared C/C++ runtime should be used. There
are some restrictions how this feature can be used, for example
on some compilers an application using static runtime should
not use shared libraries at all, and on some compilers,
mixing static and shared runtime requires extreme care. Check
your compiler documentation for more details.

threading

Allowed values:single,
multi

Controls if the project should be built in multi-threaded mode. This feature does not
necessary change code generation in the compiler, but it causes the compiler to link
to additional or different runtime libraries, and define additional preprocessor
symbols (for example, _MT on Windows and _REENTRANT on Linux).
How those symbols affect the compiled code depends on the code itself.

source

The <source>X feature has the same effect on
building a target as putting X in the list of sources. It is useful
when you want to add the same source to all targets in the project
(you can put <source> in requirements) or to conditionally
include a source (using conditional requirements, see the section called “Conditions and alternatives”). See also the <library>
feature.

library

This feature is almost equivalent to the <source>
feature, except that it takes effect only for linking. When you want
to link all targets in a Jamfile to certain library, the
<library> feature is preferred over
<source>X -- the latter will add the library to
all targets, even those that have nothing to do with libraries.

dependency

Introduces a dependency on the target named by the value of this
feature (so it will be brought up-to-date whenever the target being
declared is). The dependency is not used in any other way.

use

Introduces a dependency on the target named by the value of this
feature (so it will be brought up-to-date whenever the target being
declared is), and adds its usage requirements to the build
properties
of the target being declared. The dependency is not used in any
other way. The primary use case is when you want the usage
requirements (such as #include paths) of some library
to be applied, but do not want to link to it.

Allowed values:true, false. This property is
specific to Unix systems. If an executable is built with
<hardcode-dll-paths>true, the generated binary
will contain the list of all the paths to the used shared libraries.
As the result, the executable can be run without changing system
paths to shared libraries or installing the libraries to system
paths. This is very
convenient during development. Plase see the FAQ entry for details. Note that on Mac
OSX, the paths are unconditionally hardcoded by the linker, and it
is not possible to disable that behaviour.

cflags, cxxflags, linkflags

The value of those features is passed without modification to the
corresponding tools. For cflags that is both the C and
C++ compilers, for cxxflags that is the C++ compiler
and for linkflags that is the linker. The features are
handy when you are trying to do something special that cannot be
achieved by a higher-level feature in Boost.Build.

include

Specifies an additional include path that is to be passed to C and
C++ compilers.

warnings

The <warnings> feature controls the warning level
of compilers. It has the following values:

off - disables all warnings.

on - enables default warning level for the tool.

all - enables all warnings.

Default value is all.

warnings-as-errors

The <warnings-as-errors> makes it possible to
treat warnings as errors and abort compilation on a warning. The
value on enables this behaviour. The default value is
off.

build

Allowed values:no

The build feature is used to conditionally disable
build of a target. If <build>no is in properties
when building a target, build of that target is skipped. Combined
with conditional requirements this allows you to skip building some
target in configurations where the build is known to fail.

tag

The tag feature is used to customize
the name of the generated files. The value should have the form:

@rulename

where
rulename should be a name of a rule with the
following signature:

rule tag ( name : type ? : property-set )

The rule will be called for each target with the default name computed
by Boost.Build, the type of the target, and property set. The rule can
either return a string that must be used as the name of the target, or
an empty string, in which case the default name will be used.

Most typical use of the tag feature is to
encode build properties, or library version in library target names. You
should take care to return non-empty string from the tag rule only for
types you care about — otherwise, you might end up modifying
names of object files, generated header file and other targets for which
changing names does not make sense.

debug-symbols

Allowed values:on, off.

The debug-symbols feature specifies if
produced object files, executables and libraries should include
debug information.
Typically, the value of this feature is implicitly set by the
variant feature, but it can be explicitly
specified by the user. The most common usage is to build
release variant with debugging information.

target-os

The operating system for which the code is to be generated. The
compiler you used should be the compiler for that operating
system. This option causes Boost.Build to use naming conventions
suitable for that operating system, and adjust build process
accordingly. For example, with gcc, it controls if import
libraries are produced for shared libraries or not.

The architecture features specifies
the general processor familty to generate code for.

instruction-set

Allowed values: depend on the used
toolset.

The instruction-set specifies for which
specific instruction set the code should be generated. The
code in general might not run on processors with older/different
instruction sets.

While Boost.Build allows a large set of possible values
for this features, whether a given value works depends on which
compiler you use. Please see
the section called “C++ Compilers” for details.

address-model

Allowed values:32, 64.

The address-model specifies if 32-bit or
64-bit code should be generated by the compiler. Whether this feature
works depends on the used compiler, its version, how the compiler is
configured, and the values of the architectureinstruction-set
features. Please see the section called “C++ Compilers”
for details.

c++-template-depth

Allowed values: Any positive
integer.

This feature allows configuring a C++ compiler with the maximal
template instantiation depth parameter. Specific toolsets may or may
not provide support for this feature depending on whether their
compilers provide a corresponding command-line option.

Note: Due to some internal details
in the current Boost Build implementation it is not possible to have
features whose valid values are all positive integer. As a
workaround a large set of allowed values has been defined for this
feature and, if a different one is needed, user can easily add it by
calling the feature.extend rule.

embed-manifest

Allowed values: on, off.

This feature is specific to the msvc toolset (see
the section called “Microsoft Visual C++”),
and controls whether the manifest files should be embedded inside
executables and shared libraries, or placed alongside them. This
feature corresponds to the IDE option found in the project settings dialog,
under Configuration Properties → Manifest Tool → Input and Output → Embed manifest.

Builtin tools

Boost.Build comes with support for a large number of C++ compilers,
and other tools. This section documents how to use those tools.

Before using any tool, you must declare your intention, and possibly
specify additional information about the tool's configuration. This is
done by calling the using rule, typically in your
user-config.jam, for example:

using gcc ;

additional parameters can be passed just like for other rules, for example:

using gcc : 4.0 : g++-4.0 ;

The options that can be passed to each tool are documented in the
subsequent sections.

This section lists all Boost.Build modules that support C++
compilers and documents how each one can be initialized. The name
of support module for compiler is also the value for
the toolset feature that can be used to explicitly
request that compiler.

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the version is not explicitly specified, it will be
automatically detected by running the compiler with the -v
option. If the command is not specified, the g++
binary will be searched in PATH.

The following options can be provided, using <option-name>option-value syntax:

cflags

Specifies additional compiler flags that will be used when
compiling C sources.

cxxflags

Specifies additional compiler flags that will be used when
compiling C++ sources.

compileflags

Specifies additional compiler flags that will be used when
compiling both C and C++ sources.

linkflags

Specifies additional command line options that will be
passed to the linker.

root

Specifies root directory of the compiler installation.
This option is necessary only if it is not possible to detect this
information from the compiler command—for example if the specified
compiler command is a user script.

rc

Specifies the resource compiler command
that will be used with the version of gcc that is being
configured. This setting makes sense only for Windows and only
if you plan to use resource files. By
default windres will be used.

rc-type

Specifies the type of resource compiler. The value can
be either windres for msvc resource compiler,
or rc for borland's resource compiler.

In order to compile 64-bit applications, you have to specify
address-model=64, and the instruction-set
feature should refer to a 64 bit processor. Currently, those
include nocona, opteron,
athlon64 and athlon-fx.

Apple Darwin gcc

The darwin module supports the version of gcc that is
modified and provided by Apple. The configuration is essentially identical
to that of the gcc module.

The darwin toolset can generate so called "fat"
binaries—binaries that can run support more than one
architecture, or address mode. To build a binary that can run both
on Intel and PowerPC processors, specify
architecture=combined. To build a binary that can run
both in 32-bit and 64-bit modes, specify
address-model=32_64. If you specify both of those
properties, a "4-way" fat binary will be generated.

Microsoft Visual C++

The msvc module supports the
Microsoft Visual
C++ command-line tools on Microsoft Windows. The supported
products and versions of command line tools are listed below:

Visual Studio 2008—9.0

Visual Studio 2005—8.0

Visual Studio .NET 2003—7.1

Visual Studio .NET—7.0

Visual Studio 6.0, Service Pack 5—6.5

The msvc module is initialized using the following
syntax:

using msvc : [version] : [c++-compile-command] : [compiler options] ;

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the version is not explicitly specified, the most recent
version found in the registry will be used instead. If the special
value all is passed as the version, all versions found in
the registry will be configured. If a version is specified, but the
command is not, the compiler binary will be searched in standard
installation paths for that version, followed by PATH.

The compiler command should be specified using forward slashes,
and quoted.

The following options can be provided, using <option-name>option-value syntax:

cflags

Specifies additional compiler flags that will be used when
compiling C sources.

cxxflags

Specifies additional compiler flags that will be used when
compiling C++ sources.

compileflags

Specifies additional compiler flags that will be used when
compiling both C and C++ sources.

linkflags

Specifies additional command line options that will be
passed to the linker.

assembler

The command that compiles assembler sources. If
not specified, ml will be used. The command
will be invoked after the setup script was executed and adjusted
the PATH variable.

compiler

The command that compiles C and C++ sources. If
not specified, cl will be used. The command
will be invoked after the setup script was executed and adjusted
the PATH variable.

compiler-filter

Command through which to pipe the output of
running the compiler. For example to pass the output to STLfilt.

idl-compiler

The command that compiles Microsoft COM interface
definition files. If not specified, midl will
be used. The command will be invoked after the setup script was
executed and adjusted the PATH variable.

linker

The command that links executables and dynamic
libraries. If not specified, link will be used.
The command will be invoked after the setup script was executed
and adjusted the PATH variable.

mc-compiler

The command that compiles Microsoft message
catalog files. If not specified, mc will be
used. The command will be invoked after the setup script was
executed and adjusted the PATH variable.

resource-compiler

The command that compiles resource files. If not
specified, rc will be used. The command will be
invoked after the setup script was executed and adjusted the
PATH variable.

setup

The filename of the global environment setup
script to run before invoking any of the tools defined in this
toolset. Will not be used in case a target platform specific
script has been explicitly specified for the current target
platform. Used setup script will be passed the target platform
identifier (x86, x86_amd64, x86_ia64, amd64 or ia64) as a
arameter. If not specified a default script is chosen based on the
used compiler binary, e.g. vcvars32.bat or
vsvars32.bat.

setup-amd64, setup-i386, setup-ia64

The filename of the target platform specific
environment setup script to run before invoking any of the tools
defined in this toolset. If not specified the global environment
setup script is used.

64-bit support

Starting with version 8.0, Microsoft Visual Studio can
generate binaries for 64-bit processor, both 64-bit flavours of x86
(codenamed AMD64/EM64T), and Itanium (codenamed IA64). In addition,
compilers that are itself run in 64-bit mode, for better
performance, are provided. The complete list of compiler
configurations are as follows (we abbreviate AMD64/EM64T to just
AMD64):

32-bit x86 host, 32-bit x86 target

32-bit x86 host, 64-bit AMD64 target

32-bit x86 host, 64-bit IA64 target

64-bit AMD64 host, 64-bit AMD64 target

64-bit IA64 host, 64-bit IA64 target

The 32-bit host compilers can be always used, even on 64-bit
Windows. On the contrary, 64-bit host compilers require both 64-bit
host processor and 64-bit Windows, but can be faster. By default,
only 32-bit host, 32-bit target compiler is installed, and
additional compilers need to be installed explicitly.

To use 64-bit compilation you should:

Configure you compiler as usual. If you provide a
path to the compiler explicitly, provide the path to the 32-bit
compiler. If you try to specify the path to any of 64-bit
compilers, configuration will not work.

When compiling, use address-model=64,
to generate AMD64 code.

To generate IA64 code, use
architecture=ia64

The (AMD64 host, AMD64 target) compiler will be used
automatically when you are generating AMD64 code and are running
64-bit Windows on AMD64. The (IA64 host, IA64 target) compiler will
never be used, since nobody has an IA64 machine to test.

It is believed that AMD64 and EM64T targets are essentially
compatible. The compiler options /favor:AMD64 and
/favor:EM64T, which are accepted only by AMD64
targeting compilers, cause the generated code to be tuned to a
specific flavor of 64-bit x86. Boost.Build will make use of those
options depending on the value of theinstruction-set
feature.

Intel C++

The intel-linux and intel-win modules
support the Intel C++ command-line compiler—the Linux
and
Windows versions respectively.

This statement may be repeated several times, if you want to configure several versions of the compiler.

If compiler command is not specified, then Boost.Build will
look in PATH for an executable icpc
(on Linux), or icc.exe (on Windows).

The following options can be provided, using <option-name>option-value syntax:

cflags

Specifies additional compiler flags that will be used when
compiling C sources.

cxxflags

Specifies additional compiler flags that will be used when
compiling C++ sources.

compileflags

Specifies additional compiler flags that will be used when
compiling both C and C++ sources.

linkflags

Specifies additional command line options that will be
passed to the linker.

The Linux version supports the following additional options:

root

Specifies root directory of the compiler installation.
This option is necessary only if it is not possible to detect this
information from the compiler command—for example if the specified
compiler command is a user script.

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the command is not specified, Boost.Build will search for
a binary named como in
PATH.

The following options can be provided, using <option-name>option-value syntax:

cflags

Specifies additional compiler flags that will be used when
compiling C sources.

cxxflags

Specifies additional compiler flags that will be used when
compiling C++ sources.

compileflags

Specifies additional compiler flags that will be used when
compiling both C and C++ sources.

linkflags

Specifies additional command line options that will be
passed to the linker.

Before using the Windows version of the compiler, you need to
setup necessary environment variables per compiler's documentation. In
particular, the COMO_XXX_INCLUDE variable should be
set, where XXX corresponds to the used backend C
compiler.

Code Warrior

The cw module support CodeWarrior compiler,
originally produced by Metrowerks and presently developed by
Freescale. Boost.Build supports only the versions of the compiler that
target x86 processors. All such versions were released by Metrowerks
before aquisition and are not sold any longer. The last version known
to work is 9.4.

The module is initialized using the following syntax:

using cw : [version] : [c++-compile-command] : [compiler options] ;

This statement may be repeated several times, if you want to configure several versions of the compiler.

If the command is not specified, Boost.Build will search for a
binary named mwcc in default installation paths and
in PATH.

The following options can be provided, using <option-name>option-value syntax:

cflags

Specifies additional compiler flags that will be used when
compiling C sources.

cxxflags

Specifies additional compiler flags that will be used when
compiling C++ sources.

compileflags

Specifies additional compiler flags that will be used when
compiling both C and C++ sources.

linkflags

Specifies additional command line options that will be
passed to the linker.

root

Specifies root directory of the compiler installation.
This option is necessary only if it is not possible to detect this
information from the compiler command—for example if the specified
compiler command is a user script.

setup

The command that sets up environment variables
prior to invoking the compiler. If not specified,
cwenv.bat alongside the compiler binary
will be used.

compiler

The command that compiles C and C++ sources.
If not specified, mwcc will be used. The
command will be invoked after the setup script was
executed and adjusted the PATH variable.

linker

The command that links executables and dynamic
libraries.
If not specified, mwld will be used. The
command will be invoked after the setup script was
executed and adjusted the PATH variable.

Third-party libraries

Boost.Build provides special support for some
third-party C++ libraries, documented below.

STLport library

The STLport library
is an alternative implementation of C++ runtime library. Boost.Build
supports using that library on Windows platfrom. Linux is
hampered by different naming of libraries in each STLport
version and is not officially supported.

Before using STLport, you need to configure it in
user-config.jam using the following syntax:

using stlport : [version] : header-path : [library-path] ;

Where version is the version of
STLport, for example 5.1.4,
headers is the location where
STLport headers can be found, and libraries
is the location where STLport libraries can be found.
The version should always be provided, and the library path should
be provided if you're using STLport's implementation of
iostreams. Note that STLport 5.* always uses its own iostream
implementation, so the library path is required.

When STLport is configured, you can build with STLport by
requesting stdlib=stlport on the command line.

Build process

The general overview of the build process was given in the
user documentation.
This section provides additional details, and some specific rules.

To recap, building a target with specific properties includes the
following steps:

applying default build,

selecting the main target alternative to use,

determining "common" properties,

building targets referred by the sources list and
dependency properties,

adding the usage requirements produces when building
dependencies to the "common" properties,

building the target using generators,

computing the usage requirements to be returned.

Alternative selection

When there are several alternatives, one of them must be
selected. The process is as follows:

For each alternative condition is defined as
the set of base properties in requirements. [Note: it might be
better to specify the condition explicitly, as in conditional
requirements].

An alternative is viable only if all properties in condition
are present in build request.

If there's one viable alternative, it's choosen. Otherwise,
an attempt is made to find one best alternative. An alternative
a is better than another alternative b, iff the set of properties
in b's condition is a strict subset of the set of properities of
'a's condition. If there's one viable alternative, which is
better than all others, it's selected. Otherwise, an error is
reported.

Determining common properties

The "common" properties is a somewhat artificial term. Those are
the intermediate property set from which both the build request for
dependencies and properties for building the target are derived.

Since default build and alternatives are already handled, we have
only two inputs: build requests and requirements. Here are the rules
about common properties.

Non-free feature can have only one
value

A non-conditional property in requirement in always
present in common properties.

A property in build request is present in
common properties, unless (2) tells otherwise.

If either build request, or requirements (non-conditional
or conditional) include an expandable property (either composite,
or property with specified subfeature value), the behaviour is
equivalent to explicitly adding all expanded properties to build
request or requirements.

If requirements include a conditional property, and
condiiton of this property is true in context of common
properties, then the conditional property should be in common
properties as well.

If no value for a feature is given by other rules
here, it has default value in common properties.

Those rules are declarative, they don't specify how to compute the
common properties. However, they provide enough information for the
user. The important point is the handling of conditional
requirements. The condition can be satisfied either by property in
build request, by non-conditional requirements, or even by another
conditional property. For example, the following example works as
expected:

Definitions

Features and properties

A feature is a normalized (toolset-independent)
aspect of a build configuration, such as whether inlining is
enabled. Feature names may not contain the '>'
character.

Each feature in a build configuration has one or more
associated values. Feature values for non-free features
may not contain the '<', ':', or
'=' characters. Feature values for free features may not
contain the '<' character.

A property is a (feature,value) pair, expressed as
<feature>value.

A subfeature is a feature that only exists in the
presence of its parent feature, and whose identity can be derived
(in the context of its parent) from its value. A subfeature's
parent can never be another subfeature. Thus, features and their
subfeatures form a two-level hierarchy.

A value-string for a feature F is a string of
the form
value-subvalue1-subvalue2...-subvalueN, where
value is a legal value for F and
subvalue1...subvalueN are legal values of some
of F's subfeatures. For example, the properties
<toolset>gcc <toolset-version>3.0.1 can be
expressed more conscisely using a value-string, as
<toolset>gcc-3.0.1.

A property set is a set of properties (i.e. a
collection without duplicates), for instance:
<toolset>gcc <runtime-link>static.

A property path is a property set whose elements have
been joined into a single string separated by slashes. A property
path representation of the previous example would be
<toolset>gcc/<runtime-link>static.

A build specification is a property set that fully
describes the set of features used to build a target.

Property Validity

For free
features, all values are valid. For all other features,
the valid values are explicitly specified, and the build
system will report an error for the use of an invalid
feature-value. Subproperty validity may be restricted so
that certain values are valid only in the presence of
certain other subproperties. For example, it is possible
to specify that the <gcc-target>mingw
property is only valid in the presence of
<gcc-version>2.95.2.

Feature Attributes

Each feature has a collection of zero or more of the following
attributes. Feature attributes are low-level descriptions of how the
build system should interpret a feature's values when they appear in
a build request. We also refer to the attributes of properties, so
that an incidental property, for example, is
one whose feature has the incidental
attribute.

incidental

Incidental features are assumed not to affect build
products at all. As a consequence, the build system may use
the same file for targets whose build specification differs
only in incidental features. A feature that controls a
compiler's warning level is one example of a likely
incidental feature.

Non-incidental features are assumed to affect build
products, so the files for targets whose build specification
differs in non-incidental features are placed in different
directories as described in "target paths" below. [ where? ]

propagated

Features of this kind are
propagated to dependencies. That is, if a main target is built using a
propagated
property, the build systems attempts to use the same property
when building any of its dependencies as part of that main
target. For instance, when an optimized exectuable is
requested, one usually wants it to be linked with optimized
libraries. Thus, the <optimization> feature is
propagated.

free

Most features have a finite set of allowed values, and can
only take on a single value from that set in a given build
specification. Free features, on the other hand, can have
several values at a time and each value can be an arbitrary
string. For example, it is possible to have several
preprocessor symbols defined simultaneously:

<define>NDEBUG=1 <define>HAS_CONFIG_H=1

optional

An optional feature is a feature that is not required to
appear in a build specification. Every non-optional non-free
feature has a default value that is used when a value for
the feature is not otherwise specified, either in a target's
requirements or in the user's build request. [A feature's
default value is given by the first value listed in the
feature's declaration. -- move this elsewhere - dwa]

symmetric

A symmetric feature's default value is not automatically
included in build variants. Normally
a feature only generates a subvariant directory when its
value differs from the value specified by the build variant,
leading to an assymmetric subvariant directory structure for
certain values of the feature. A symmetric feature, when
relevant to the toolset, always generates a corresponding
subvariant directory.

path

The value of a path feature specifies a path. The path is
treated as relative to the directory of Jamfile where path
feature is used and is translated appropriately by the build
system when the build is invoked from a different
directory

implicit

Values of implicit features alone identify the feature.
For example, a user is not required to write
"<toolset>gcc", but can simply write "gcc". Implicit
feature names also don't appear in variant paths, although
the values do. Thus: bin/gcc/... as opposed to
bin/toolset-gcc/.... There should typically be only a few
such features, to avoid possible name clashes.

composite

Composite features actually correspond to groups of
properties. For example, a build variant is a composite
feature. When generating targets from a set of build
properties, composite features are recursively expanded and
added to the build property set, so rules can find
them if necessary. Non-composite non-free features override
components of composite features in a build property set.

dependency

The value of a dependency feature is a target reference.
When used for building of a main target, the value of
dependency feature is treated as additional dependency.

For example, dependency features allow to state that
library A depends on library B. As the result, whenever an
application will link to A, it will also link to B.
Specifying B as dependency of A is different from adding B to
the sources of A.

Features that are neither free nor incidental are called
base features.

Feature Declaration

The low-level feature declaration interface is the
feature rule from the
feature module:

rule feature ( name : allowed-values * : attributes * )

A feature's allowed-values may be extended with the
feature.extend rule.

Build Variants

A build variant, or (simply variant) is a special kind of composite
feature that automatically incorporates the default values of
features that . Typically you'll want at least two separate
variants: one for debugging, and one for your release code. [
Volodya says: "Yea, we'd need to mention that it's a composite
feature and describe how they are declared, in pacticular that
default values of non-optional features are incorporated into
build variant automagically. Also, do we wan't some variant
inheritance/extension/templates. I don't remember how it works in
V1, so can't document this for V2.". Will clean up soon -DWA ]

Property refinement

When a target with certain properties is requested, and that
target requires some set of properties, it is needed to find the
set of properties to use for building. This process is called
property refinement and is performed by these rules

Each property in the required set is added to the original
property set

If the original property set includes property with a different
value of non free feature, that property is removed.

Conditional properties

Sometime it's desirable to apply certain requirements only for
a specific combination of other properties. For example, one of
compilers that you use issues a pointless warning that you want to
suppress by passing a command line option to it. You would not
want to pass that option to other compilers. Conditional
properties allow you to do just that. Their syntax is: